1. Field of the Invention
The present invention relates to semiconductor integrated circuit structures and methods of making such structures and, more particularly, to method of making and structure of an SRAM cell having N channel depletion TFT load devices.
2. Prior Art
CMOS static RAM access memories (SRAM) are used in the semiconductor and computer industries as a result of the combination of speed, low power, and no requirement for refresh. Information can be written into and read out of an SRAM cell faster than with a DRAM cell, because the transistors of the SRAM cell can be switched faster than capacitors can be charged and drained. However, a disadvantage of prior art SRAM cells is that such cells have required a larger footprint to achieve greater speed and stability than DRAM cells.
An SRAM cell can be formed using cross-coupled CMOS inverters having two N channel transistors and two P channel transistors. Typically, the cell is accessed by two N channel control gates for a standard SRAM cell and four control gates for two port memory devices.
There have been many attempts to improve SRAM cells by replacing the P channel transistors with other devices. For example, in some cases, the P channel transistors are replaced with poly silicon resistance back-to-back diodes as resistive load devices. However, the resistance of the back-to-back diode increases significantly at lower voltages and lower temperatures. For example, resistance might be ten times higher at 0.degree. C. as compared to 80.degree. C.
Further, a major single bit failure which has occurred during functional testing of SRAM cells is data retention at low voltage at low temperature. These single bit failures occur when the amount of leakage current at the storage node exceeds the amount of current that can be supplied by the back-to-back diode resistance during low voltage and cold temperature.
One attempt to solve the problem has been to reduce the overall resistance value of the back-to-back diode. However, when the load resistance of a cell is decreased, the amount of standby current is significantly increased, thus increasing power dissipation in the cell.
Another prior art approach has been to employ poly silicon resistance devices as load devices. Although the voltage dependency is lower than that of the back-to-back resistance diode approach, the temperature dependency still prevails with higher resistance values at low temperature as compared to high temperature resistance.
The P channel MOSFET device provides a low off current and a high on current to sustain leakage of the storage node. However, if the pulldown transistors exhibit high leakage, the Vcc must be electrically disconnected to reduce the standby current. Additionally, for the P channel device, the cell area is much larger than for the other devices described above. Such a cell and the method of making same are taught in U.S. Pat. No. 5,187,114.
Another prior art attempt to solve some of the problems of load devices in SRAM cells has been the use of P channel thin film transistors as the load devices. However, the P channel TFTs are difficult to fabricate with low off current and high on current and further requires the alignment offset of the Drain implant which has a large impact on controlling the on and off current. Further, the P channel TFT has a cell area which is at least 50 percent larger than the back-to-back diode load implementation.
There is a need for an SRAM cell which is relatively immune from voltage variation, which does not require a source/drain offset, which does not require the Vcc line to be disconnected where the pulldown transistors exhibit high leakage current, and which conserves power and energy by controlling current from low Vcc to high Vcc.